(a) Field of the Invention
This invention relates to a finder optical system comprising a field lens, field mask and eyepiece.
(b) Description of the Prior Art
A finder for photographic cameras is provided with a mask showing a photographed range, that is, a field mask. Such mask is generally rectangular. In a conventional finder device, the image of the mask may be seen as distorted.
For example, a finder for conventional single-lens reflex photographic cameras is of such formation as is shown in FIG. 1. The reference numeral 1 denotes a picture taking objective. A light having passed through this picture taking objective is reflected by a quick return mirror 2 arranged in front of a film face 3 and arrives on a focussing glass on which an image is formed. Further, the light passes through a field lens 6 and pentagonal roof prism 7 and is observed with an eyepiece 8. In such finder optical system, the light from the mask 4 showing a photographed range reaches the eyepiece through the field lens 6 and is observed. Therefore, if there is a distortion in the eyepiece system including the field lens 6, the image of the mask will be seen as distorted to be bobbin-shaped, because, as shown in FIG. 2, if the image magnified by the eyepiece is considered to be an object and the real image 9 is considered to be an object image and is considered as reversely traced from the side of the exit pupil 10, as the eyepiece 8 and field lens 6 are both convex lenses, a strong negative distortion will occur and, as the ray height of the principal ray is low in the eyepiece but is high in the field lens, the distortion will occur mostly in the field lens. In fact, as the ray proceeds toward the eyepiece 8 from the real image 9 side, the finder optical system will have a strong positive distortion and such rectangular mask as in FIG. 3 will be seen to be bobbin-shaped as in FIG. 4 to give an ugly queer feeling.